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March 03, 2015

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First-ever photo said to capture light acting as both particle and wave

March 2, 2015
Courtesy of EPFL
and World Science staff

Quan­tum me­chan­ics, the phys­ics of sub­a­tom­ic par­t­i­cles, tells us that light can be­have sim­ul­ta­ne­ously as a par­t­i­cle or a wave. 

But there has nev­er been an ex­pe­ri­ment able to cap­ture both na­tures of light at the same time; the clos­est we have come is see­ing ei­ther wave or par­t­i­cle, but al­ways at dif­fer­ent times. 

Energy-space pho­tog­ra­phy of light con­fined on a nanowire, si­mul­ta­ne­ous­ly show­ing both spa­tial in­ter­fer­ence and en­er­gy quan­ti­za­tion, ac­cord­ing to re­search­ers. (Cred­it: Fab­rizio Car­bone/EPFL)


Tak­ing a new ap­proach, sci­en­tists now say they have man­aged to take the first snap­shot of light be­hav­ing in both ways. 

The re­sults are pub­lished in the jour­nal Na­ture Com­mu­nica­t­ions.

When ul­tra­vi­o­let light hits a met­al sur­face, it knocks elec­tric­ally charged bits of atoms, called elec­trons, out of the sur­face. Al­bert Ein­stein ex­plained this “photoe­lec­tric” ef­fect by pro­pos­ing that light—thought to only be a wave—is al­so a stream of par­t­i­cles. 

A va­ri­e­ty of ex­pe­ri­ments since then have suc­cess­fully ob­served both the par­t­i­cle- and wave-like be­hav­iors, with­out cap­tur­ing both at the same time.

In the new work, re­search­ers led by Fab­rizio Car­bone at Ecole Poly­tech­nique Fédérale de Lau­sanne in Switz­er­land car­ried out an ex­pe­ri­ment with a twist: us­ing elec­trons to im­age light. 

The ex­pe­ri­ment is set up like this. A pulse of la­ser light is fired at an ex­tremely thin wire, or “nanowire.” The la­ser adds en­er­gy to the charged par­t­i­cles in the nanowire, caus­ing them to vi­brate. Light trav­els along this ti­ny wire in two pos­si­ble di­rec­tions, like cars on a high­way. When waves trav­el­ing in op­po­site di­rec­tions meet each oth­er they form a new wave that looks like it is stand­ing in place. 

This stand­ing wave be­comes the source of light for the ex­pe­ri­ment, ra­di­at­ing around the nanowire.

Having done this, the sci­en­tists next shot a stream of elec­trons close to the nanowire, us­ing them to im­age the stand­ing wave. As the elec­trons in­ter­acted with the con­fined light on the nanowire, the elec­trons ei­ther sped up or slowed down, the re­search­ers ex­plained. Us­ing an ul­trafast mi­cro­scope to im­age where this change in speed oc­curred, Car­bone’s team could now “see” the stand­ing wave, which acts as a fin­ger­print of the wave-nature of light.

But this sighting dem­on­strat­ed the par­t­i­cle as­pect as well, the sci­en­tists main­tain. As the elec­trons pass close to the stand­ing wave of light, they “hit” the light’s par­t­i­cles, called pho­tons. This makes them move faster or slower. This change in speed ap­pears as an ex­change of en­er­gy “pack­ets” called quan­ta be­tween elec­trons and pho­tons. The very oc­cur­rence of these en­er­gy pack­ets shows that the light on the nanowire be­haves as a par­t­i­cle, ac­cord­ing to the in­ves­ti­ga­tors.

“This ex­pe­ri­ment demon­strates that, for the first time ev­er, we can film quan­tum me­chan­ics—and its par­a­dox­i­cal na­ture—di­rect­ly,” said Car­bone. The work’s im­por­tance can ex­tend to fu­ture tech­nolo­gies, he added: “Be­ing able to im­age and con­trol quan­tum phe­nom­e­na at the na­no­me­ter scale like this opens up a new route to­wards quan­tum com­put­ing,” or ultra­fast com­puters that exploit quan­tum effects.

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Quantum mechanics, the physics of subatomic particles, tells us that light can behave simultaneously as a particle or a wave. But there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a new approach, scientists now say they have managed to take the first snapshot of light behaving in both ways. The results are published in the journal Nature Communications. When ultraviolet light hits a metal surface, it knocks electrically charged bits of atoms, called electrons, out of the surface. Albert Einstein explained this “photoelectric” effect by proposing that light—thought to only be a wave—is also a stream of particles. A variety of experiments since then have successfully observed both the particle- and wave-like behaviors, without capturing both at the same time. In the new work, researchers led by Fabrizio Carbone at Ecole Polytechnique Fédérale de Lausanne in Switzerland carried out an experiment with a twist: using electrons to image light. The experiment is set up like this: A pulse of laser light is fired at an extremely thin wire, or “nanowire.” The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. This standing wave becomes the source of light for the experiment, radiating around the nanowire. Next the scientists shot a stream of electrons close to the nanowire, using them to image the standing wave. As the electrons interacted with the confined light on the nanowire, the electrons either sped up or slowed down, the researchers explained. Using an ultrafast microscope to image where this change in speed occurred, Carbone’s team could now “see” the standing wave, which acts as a fingerprint of the wave-nature of light. But this demonstrated the particle aspect as well, the scientists maintain. As the electrons pass close to the standing wave of light, they “hit” the light’s particles, called photons. This makes them move faster or slower. This change in speed appears as an exchange of energy “packets” called quanta between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle, according to the investigators. “This experiment demonstrates that, for the first time ever, we can film quantum mechanics—and its paradoxical nature—directly,” said Carbone. The work’s importance can extend to future technologies, he added: “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”